Here, the union's constructor is defined to activate its empty data member _M_rest, but during constexpr evaluation of this constructor the subobject constructor call to O::O(&_M_rest, 42) produces no side effects that actually activates the member, so the union still appears uninitialized after the fact. This patch fixes this by faking up a dummy MODIFY_EXPR in this situation, whose evaluation ensures the member gets activated.
Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for trunk? PR c++/102163 gcc/cp/ChangeLog: * constexpr.c (cxx_eval_call_expression): After evaluating a constructor call for an empty union member, produce a side effect that makes sure the member is activated. gcc/testsuite/ChangeLog: * g++.dg/cpp0x/constexpr-empty17.C: New test. --- gcc/cp/constexpr.c | 34 +++++++++++++++---- .../g++.dg/cpp0x/constexpr-empty17.C | 21 ++++++++++++ 2 files changed, 49 insertions(+), 6 deletions(-) create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-empty17.C diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c index 7772fe62d95..40b0b80b438 100644 --- a/gcc/cp/constexpr.c +++ b/gcc/cp/constexpr.c @@ -2787,12 +2787,34 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t, &jump_target); if (DECL_CONSTRUCTOR_P (fun)) - /* This can be null for a subobject constructor call, in - which case what we care about is the initialization - side-effects rather than the value. We could get at the - value by evaluating *this, but we don't bother; there's - no need to put such a call in the hash table. */ - result = lval ? ctx->object : ctx->ctor; + { + /* This can be null for a subobject constructor call, in + which case what we care about is the initialization + side-effects rather than the value. We could get at the + value by evaluating *this, but we don't bother; there's + no need to put such a call in the hash table. */ + result = lval ? ctx->object : ctx->ctor; + + if (!result && new_obj + && TREE_CODE (new_obj) == COMPONENT_REF + && TREE_CODE (TREE_TYPE + (TREE_OPERAND (new_obj, 0))) == UNION_TYPE + && is_really_empty_class (TREE_TYPE (new_obj), + /*ignore_vptr*/false)) + { + /* This constructor call for an empty union member might not + have produced a side effect that actually activated the + member. So produce such a side effect now to ensure the + union appears initialized. */ + tree activate = build2 (MODIFY_EXPR, TREE_TYPE (new_obj), + new_obj, + build_constructor (TREE_TYPE (new_obj), + NULL)); + cxx_eval_constant_expression (ctx, activate, lval, + non_constant_p, overflow_p); + ggc_free (activate); + } + } else if (VOID_TYPE_P (TREE_TYPE (res))) result = void_node; else diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-empty17.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-empty17.C new file mode 100644 index 00000000000..9d753a3bb69 --- /dev/null +++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-empty17.C @@ -0,0 +1,21 @@ +// PR c++/102163 +// { dg-do compile { target c++11 } } + +struct O { + constexpr O(int) { } +}; + +union _Variadic_union { + constexpr _Variadic_union(int __arg) : _M_rest(__arg) { } + + int _M_first; + O _M_rest; +}; + + +struct _Variant_storage { + constexpr _Variant_storage() : _M_u(42) {} + _Variadic_union _M_u; +}; + +constexpr _Variant_storage w; -- 2.33.0.328.g8b7c11b866